Terms used in Automatic Control of Systems

Terms used in Automatic Control of Systems

The following terms are generally used in automatic control of systems: 

1. Command:  The result of the act of adjustment, i.e. closing a valve, moving a lever, pressing buttons etc., is known as Command.
2. Response: The subsequent result of the system to the command is known as response.
3. Process control: The automatic control of variables like  change in pressure, temperature and speed etc., in machine is termed as process control.
4. Process controller: The device which controls a process is called a process control.
5. Kinetic control: The automatic control of the displacement or velocity or acceleration of a member of a machine is called as Kinetic control.
6. Regulator: The device used to keep the variables at a constant desired value is called as regulator.
7. Feed back: It is defined as measuring the output of the machine for comparison with the input to the machine.
8. Error detector: A differential device used to measure the actual controlled quantity and to compare it continuously with the desired value is called an error detector. It is also called deviation sensor.
9. Transducer: It is a device to change a signal which is in one physical form to a corresponding signal in another physical form. The example of transducer are a loudspeaker (because it converts electrical signal into a sound) and photo-electric cell (because it converts a light signal into an electric signal). Similarly, the primary elements of all the many different forms of thermometers are transducers.
10. Amplification: It is defined as increasing the amplitude of the signal without affecting its waveform. For example, an error detector itself has insufficient power output to actuate the correcting mechanism and hence the error signal has to be amplified. This is generally done by employing mechanical or hydraulic or pneumatic amplifying elements like levers, gears etc.,

Block Diagrams

Block Diagrams

Fig. Block diagram of a single Carburettor.

          The block diagrams are used to study the automatic control system in simplified way. In this, the functioning of a system is explained by the inter connected blocks where each block represents a labelled rectangle and is thought of as a block box with a definite function. These blocks are connected to other blocks by lines with arrow marks in order to indicate the sequence of events that are taking place. It may also show how the system operates, what are its inputs and outputs at various stages, and how the energy, information, and/or materials flow through it. Above Fig., shows the diagram of a simple carburattor. The reduction of a control system to a block diagram greatly facilitates the analysis of the system performance or response.

Lag in Response

Lag in Response

          We know that response is the subsequent result of the system to the command. In any control system, there is a delay in response (output) due to some inherent cause and it becomes difficult to measure the input and output simultaneously. This delay in response is termed as lag in response. For example, in steam turbine, with the sudden decrease in load, the hydraulic relay moves in the direction to close the valve. But unless the piston valve ports are made with literally zero overlap, there would be some lag in operation, since the first movement of the piston valve would not be sufficient to open the ports. This lag increases the probability of unstable operation.

Damping

Damping

          When torque is applied in a system in a direction opposite to its motion, it is known as Damping. In case of coulomb damping, the opposition is constant and, thus there will be a constant difference between the input and the output under steady conditions. In the viscous damping provided by dashpot, the opposition is proportional to the relative velocity. As the relative velocity is zero in the steady state, the damping is also zero.

Transfer Function

Transfer Function

          The transfer function is a mathematical expression showing the relation between output and the input to  each unit or block of a control system. If the system has a single input and a single output, it can be represented by block diagram as shown in Fig., 

Transfer function is defined as the ratio of output over the input with all initial conditions equal to zero. Mathematically, 

Transfer function =  θ0 / θ_i

              Where      θ0 = Output signal of the block of a system, and 

                               θ_i  = Input signal to the block of a system.

Thus, the output from an element be obtained by multiplying the input signal with the transfer function.

Note : From the transfer function of the individual block, the equation of motion of system can be formulated.

Overall Transfer Function

Overall Transfer Function

          In the previous topic, we have discussed the transfer function of a block. In this we are going to know the Overall transfer function of a control system. A control system actually consists of several such blocks which are connected in series. The overall transfer function of the series is the product of the individual transfer functions. Consider a block diagram of any control system represented by the three blocks as shown in Fig., 

Fig. Overall transfer function.

          Thus, if  are individual transfer functions of three blocks in series, then th overall transfer function of the system is given as 

Where       K = Constant representing the overall amplification or gain, and 

                  G(D) = Some function of the operator D.

Note: The above equation only true if there is no interaction between the blocks, that is the output from one block is not affected by its connection to the subsequent blocks.